Composition for the fluorescent whitening of paper

ABSTRACT

A composition comprising a) 5 to 25% by weight of a fluorescent whitening agent of the formula (1), wherein R 1  and R 2  independently represent C 1 -C 4 alkyl, C 2 -C 4 hydroxyalkyl or together complete a morpholine ring and M is an alkaline or alkaline earth metal, b) 0.5 to 5% by weight of a swellable layered silicate which is hectorite and/or vermiculite, c) water and, optionally, d) further adjuvants and the use thereof for the fluorescent whitening of paper in the pulp mass, in coatings or in the size press or metering size press.

[0001] The present invention relates to a composition useful for the fluorescent whitening of paper in the pulp mass, in coatings or in the size press or metering size press.

[0002] Aqueous slurries of fluorescent whitening agents (FWA's) have been found to be advantageous in their applications in the paper industry, both from a technical and from an ecological point of view, as opposed to solutions, which necessitate considerable quantities of solvating agents such as urea or solvents such as polyalkylene glycols.

[0003] However, such aqueous slurries must be stable against sedimentation, serum formation and also to storage at elevated temperatures. Such stabilization has previously been achieved by the addition of polymers, such as xanthogenates and synthetic water-soluble polymers, or dispersing agents.

[0004] Surprisingly, it has now been found that particular smectic, layered silicates, namely hectorite and/or vermiculite, exhibit an excellent stabilizing influence on such aqueous slurries.

[0005] A further problem by the application of FWA's in the pulp mass is their property, not only not to enhance, but, indeed, in some cases, to reduce the retention of fillers. It has now been found that the quantities of hectorite and/or vermiculite useful for the stabilization of aqueous slurries also exert a very positive influence on the retention of fillers. Such effects are known in the case of bentonite.

[0006] However, bentonite is not suitable for the stabilization of aqueous slurries and, furthermore, it has been found that, for example, hectorite exerts a far more efficient Increase In the retention of fillers than bentonite.

[0007] Consequently, the composition of the invention combines the advantages of these particular smectic, layered silicates with regard to their stabilizing influence of aqueous slurries with a vast improvement in filler retention, effects which are of high technical value for the paper-maker.

[0008] The subject of the present invention is, thus, a composition comprising

[0009] a) 5 to 25%, preferably 10 to 20%, by weight of a fluorescent whitening agent of the formula

[0010]  wherein R₁ and R₂ independently represent C₁-C₄alkyl, C₂-C₄hydroxyalkyl or together complete a morpholine ring and

[0011] M is an alkaline or alkaline earth metal,

[0012] b) 0.5 to 5% by weight of a swellable layered silicate which is hectorite and/or vermiculite,

[0013] c) water and, optionally,

[0014] d) further adjuvants.

[0015] In one preferred aspect of the invention, the composition comprises 0.5 to 3%, more preferably 0.5 to 1.5%, by weight of hectorite, whilst, in a further preferred aspect, the composition comprises 0.5 to 5%, more preferably 0.5 to 1.5%, by weight of vermiculite.

[0016] When, in the compound of formula (1), R₁ and/or R₂ represent C₁-C₄alkyl, these may be methyl, ethyl, n- or isopropyl or n-, iso- or t-butyl, whilst C₂-C₄hydroxyalkyl may be hydroxyethyl, hydroxypropyl or hydroxybutyl.

[0017] When, in the compound of formula (1), M is an alkaline metal, this may be lithium, potassium or sodium, whilst an alkaline earth metal may be calcium or magnesium.

[0018] Preferably, in the compound of formula (1), R₁ represents methyl, ethyl, hydroxyethyl or hydroxypropyl, R₂ represents hydroxyethyl or hydroxypropyl, or R₁ and R₂ together complete a morpholine ring and M is lithium, potassium or sodium, a most preferable composition comprising a fluorescent whitening agent selected from the compounds of formulae

[0019] Optionally, the composition of the invention may comprise further adjuvants, for example, conserving agents such as 1,2-benzisothiazolin-2-one or aldehydes such as formaldehyde or acetaldehyde, anti-freeze agents or anti-foaming agents, whereby such adjuvants are normally employed in small quantities, for example, below 1% by weight.

[0020] The composition of the invention may be obtained, for example, by intensive mixing of the FWA of formula (1), either in the form of a moist filter-cake or a dry powder, with water under high shear forces, subsequently, adding a pre-dispersion of hectorite and/or vermiculite in water and, finally stirring until a homogeneous mixture results, optionally, in the presence of appropriate adjuvants.

[0021] The resulting compositions are stable to storage over longer periods of time, for example, 6 months, even at temperatures ranging from 0° C. to 40° C.

[0022] Using the composition of the present invention, it has been found that it is possible to increase the whiteness of a lignin-containing pulp by a process which comprises adding to an aqueous slurry comprising a lignin-containing pulp, during pulp manufacture, prior to the drying step or paper making step if the pulp is not isolated, an effective amount of an FWA. While the FWA composition can be added to the aqueous slurry comprising the lignin-containing pulp at any processing step, to minimize losses, it is advantageously added in the latter stages of pulp manufacture, prior to the final de-watering and drying steps. Preferably it is added after completion of the last bleaching step.

[0023] By “a lignin-containing pulp” is meant any pulp that still contains about 5% or more of lignin by weight on a dry basis. By definition, lignin is that portion of the pulp which is insoluble in 72 weight percent sulfuric acid. Suitable test procedures for lignin content are given in TAPPI T 223 and ASTM D 1106.

[0024] The composition of this invention is useful to produce significant whitening of chemical pulps as well as pulps containing up to 100% of mechanical fibre. Thus the composition can be employed, e.g. in lignin-free pulps, such as certain bleached kraft pulps, up to and including higher lignin content pulps such as thermomechanical pulps, bleached chemi-thermomechanical pulps (CTMP), and even deinked bleached thermomechanical pulps. Preferably the pulps contain at least 10% of lignin by weight on a dry weight basis; most preferably they contain at least 15%. The range of brightness that can be obtained varies from about 50 to 90+ depending on starting pulp brightness and the type of pulp employed.

[0025] Addition of the composition of the invention to the pulp mass may be performed at any step of the paper-making process, but preferably is performed shortly before the head box.

[0026] It is known to employ chelating agents in processes to bleach pulps from mechanical pulping processes. See V. N. Gupta, Pulp Paper Mag. Can., 71 (18), T391-399 (1970). The addition of a chelating agent to an aqueous pulp slurry controls the natural yellowing tendency of glucuronic acids, extractives and lignin present in the pulp by removing or minimizing iron and other heavy metals such as copper, zinc and manganese metals that catalyze color-forming side reactions. The iron and other heavy metals are converted into the form of their highly soluble chelates and largely removed In the de-watering steps. This decreases the incorporation of the heavy metal ions into the pulp. Additionally the chelating agent sequesters the salts of iron and other heavy metals which remain and which, in their own right would otherwise relax the excited state of FWA's and render them ineffective.

[0027] All types of chelating agents are suitable in the present invention, i.e. those that offer thermodynamic or kinetic control of metal ions. However preference is given to chelating agents that offer thermodynamic control, that is, chelating agents that form a stable, isolable, complex with a heavy metal ion. Within this group it is particularly preferred to use aminocarboxylic acid chelates. Well known and commercially available members of this class include ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), hydroxyethylethylenediaminetriacetic acid (HEDTA) and nitrilotriacetic acid (NTA).

[0028] Mixtures of thermodynamic and kinetic-controlling chelating agents (e.g. citrates, keto acids, gluconates, heptagluconates, phosphates, and phosphonates) also work well in reducing the content of free heavy metal ions in the pulp to acceptable levels. A number of these kinetic-controlling chelating agents are also commercially available. Kinetic controlling chelating agents are those which do not form a stable, isolable, complex with a heavy metal ion.

[0029] When used for the fluorescent whitening of paper, the compositions according to the present invention may also be applied to the paper substrate in the form of a paper coating composition or directly in the size press.

[0030] In a further aspect, the present invention provides a method for the fluorescent whitening of a paper surface, comprising contacting the paper surface with a coating composition comprising a white pigment; a binder dispersion; optionally a water-soluble co-binder; and sufficient of a composition according to the present invention, to ensure that the treated paper contains 0.01 to 2 parts by weight, preferably 0.01 to 1 parts by weight, based on the white pigment, of the FWA (parts of the FWA per 100 parts of the pigment).

[0031] As the white pigment component of the paper coating composition used according to the method of the present invention, there are preferred inorganic pigments, e.g., aluminium or magnesium silicates, such as China clay and kaolin and, further, barium sulfate, satin white, titanium dioxide, calcium carbonate (chalk) or talcum; as well as white organic pigments.

[0032] The paper coating compositions used according to the method of the present invention may contain, as binder, inter alia, plastics dispersions based on copolymers of butadiene/styrene, acrylonitrile/butadiene/styrene, acrylic acid esters, acrylic acid esters/styrene/acrylonitrile, ethylene/vinyl chloride and ethylene/vinyl acetate; or homopolymers, such as polyvinyl chloride, polyvinylidene chloride, polyethylene and polyvinyl acetate or polyurethanes. A preferred binder consists of styrene/butyl acrylate or styrene/butadiene/acrylic acid copolymers or styrene/butadiene rubbers. Other polymer latices are described, for example, in U.S. Pat. Nos. 3,265,654, 3,657,174, 3,547,899 and 3,240,740.

[0033] The optional water-soluble protective colloid may be, e.g., soya protein, casein, carboxymethylcellulose, natural or modified starch, chitosan or a derivative thereof or, especially, polyvinyl alcohol. The preferred polyvinyl alcohol protective colloid component may have a wide range of saponification levels and molecular weights; e.g. a saponification level ranging from 40 to 100; and an average molecular weight ranging from 10,000 to 100,000.

[0034] Recipes for coating compositions for paper are described, for example, in J. P. Casey “Pulp and Paper”; Chemistry and Chemical Technology, 2nd edition, Volume III, pages1684-1649 and in “Pulp and Paper Manufacture”, 2nd and 5th edition, Volume II, page497 (McGraw-Hill).

[0035] The paper coating compositions used according to the method of the present invention preferably contain 10 to 70% by weight of a white pigment. The binder is preferably used in an amount which is sufficient to make the dry content of polymeric compound up to 1 to 30 parts by weight, preferably 5 to 25 parts by weight, of the white pigment. The amount of FWA composition used according to the invention is calculated so that the FWA is preferably present in amounts of 0.01 to 1 part by weight, more preferably 0.05 to 1 part by weight, and especially 0.05 to 0.6 parts by weight, based on the white pigment.

[0036] The paper coating composition used in the method according to the invention can be prepared by mixing the components in any desired sequence at temperatures from 10 to 100° C., preferably 20 to 80° C. The components here also include the customary auxiliaries which can be added to regulate the rheological properties, such as viscosity or water retention capacity, of the coating compositions. Such auxiliaries are, for example, natural binders, such as starch, casein, protein or gelatin, cellulose ethers, such as carboxyalkylcellulose or hydroxyalkylcellulose, alginic acid, alginates, polyethylene oxide or polyethylene oxide alkyl ethers, copolymers of ethylene oxide and propylene oxide, polyvinyl alcohol, water-soluble condensation products of formaldehyde with urea or melamine, polyphosphates or polyacrylic acid salts.

[0037] The coating composition used according to the method of the present invention is preferably used to produce coated printed or writing paper, or special papers such as cardboard or photographic papers.

[0038] The coating composition used according to the method of the invention can be applied to the substrate by any conventional process, for example with an air blade, a coating blade, a roller, a doctor blade or a rod, in the size press or in the metering size press, after which the coatings are dried at paper surface temperatures in the range from 70 to 200° C., preferably 90 to 130° C., to a residual moisture content of 3-8%, for example with infra-red dryers and/or hot-air dryers. Comparably high degrees of whiteness are thus achieved even at low drying temperatures.

[0039] By the use of the method according to the invention, the coatings obtained are distinguished by optimum distribution of the FWA over the entire surface and by an increase in the level of whiteness thereby achieved, by a high fastness to light and to elevated temperature (e.g. stability for 24 hours at 60-10° C.) and excellent bleed-fastness to water.

[0040] In a still further aspect, the present invention provides a method for the fluorescent whitening of a paper surface comprising contacting the paper in the size press or metering press with an aqueous preparation containing a size, optionally an inorganic or organic pigment and 0.1 to 20 g/l of an FWA. Preferably, the size is starch, a starch derivative or a synthetic sizing agent, especially a water-soluble copolymer.

[0041] The following examples further illustrate the invention, without intending to be restrictive in nature. Parts and percentages are by weight unless otherwise stated.

EXAMPLE 1

[0042] To 40 g of a 20% slurry formulation obtained from of 20% of the compound of formula (2) as solid crystals, 0.2% of a polysaccharide biopolymer and water, are added 10 g of a pre-dispersion prepared by the application of high shear forces to a mixture of 5 g of hectorite and 95 g of water for 15 minutes. The mixture is stirred until it is homogenous.

[0043] A formulation containing 16% of the compound (2) and 1% hectorite is obtained, which, as opposed to the starting formulation, is stable to storage.

[0044] Thus, after centrifuging at 3,500 rpm for 30 minutes, only 10% serum separation is observed as opposed to 50% for the starting 20% slurry.

[0045] Similarly, after storage at elevated temperature, 40% less serum formation is observed, in comparison to the starting 20% slurry.

EXAMPLE 2

[0046] To a fibre dispersion consisting of a mixture of birch and pine cellulose sulphates refined to 35⁰SR, are added 20% and 40%, respectively, of precipitated calcium carbonate (PCC) as filler. Then, 0.01% or 0.05% of a cationic polyacrylamide (CPAM) as retention aid (Percol® 292) are added, followed by, after 3 minutes, sufficient of the slurry obtained according to Example 1 such that, together with the FWA of formula (2), 0.04% of hectorite, based on the weight of fibre, is present.

[0047] Hand sheets are then prepared by means of the Rapid-Koethen System and the percentage retention of the filler is measured.

[0048] The results are shown in the following Table 1, below, whereby, in the absence of both CPAM and hectorite, retention corresponds to 54% with 20% PCC and 53% with 40% PCC: TABLE 1 20% PCC 0.01% 40% PCC % Hectorite CPAM 0.05% CPAM 0.01% CPAM 0.05% CPAM None 70% 71% 56% 62% 0.04 81% 88% 64% 70%

[0049] The results clearly demonstrate the improvement in the retention of PCC effected by the addition of extremely small quantities of hectorite. 

1. A composition comprising a) 5 to 25% by weight of a fluorescent whitening agent of the formula

 wherein R₁ and R₂ independently represent C₁-C₄alkyl, C₂-C₄hydroxyalkyl or together complete a morpholine ring and M is an alkaline or alkaline earth metal, b) 0.5 to 5% by weight of a swellable layered silicate which is hectorite and/or vermiculite, c) water and, optionally, d) further adjuvants.
 2. A composition according to claim 1 comprising 10 to 20% by weight of the fluorescent whitening agent of formula (1).
 3. A composition according to claims 1 or 2 comprising 0.5 to 3% by weight of hectorite.
 4. A composition according to claim 3 comprising 0.5 to 1.5% by weight of hectorite.
 5. A composition according to claims 1 or 2 comprising 0.5 to 5% by weight of vermiculite.
 6. A composition according to claim 5 comprising 0.5 to 1.5% by weight of vermiculite.
 7. A composition according to any one of claims 1 to 6, wherein, In the fluorescent whitening agent of formula (1), R₁ represents methyl, ethyl, hydroxyethyl or hydroxypropyl, R₂ represents hydroxyethyl or hydroxypropyl or R₁ and R₂ together complete a morpholine ring and M represents lithium, potassium or sodium
 8. A composition according to claim 7 wherein the fluorescent whitening agent is selected from the compounds of formulae


9. Use of the composition according to any one of claims 1 to 8 for the fluorescent whitening of paper.
 10. Use according to claim 9, whereby the composition is added to the pulp mass.
 11. Use according to claim 9, whereby the composition is used for paper coating.
 12. Use according to claim 9, whereby the composition is added to the size press or metering size press.
 13. Paper, which has been optically brightened by the method according to claims 9 to
 12. 